This invention relates generally to device metallization, its use in a semiconductor device, and to a method for its fabrication, and more specifically to a multilayered device metallization suitable for use in small geometry semiconductor devices and to methods for its fabrication.
The semiconductor industry is characterized by a trend toward fabricating larger and more complex functions on a given semiconductor chip. The larger and more complex functions are achieved by reducing device sizes and spacing and by reducing the junction depth of regions formed in the semiconductor substrate. Among the feature sizes which are reduced in size are the width and spacing of interconnecting metal lines and the contact openings through which the metallization makes electrical contact to device regions. As the feature sizes are reduced, new problems arise which must be solved in order to economically and reliably produce the semiconductor devices.
As both the contact size and junction depth are reduced, a new device metallization process is required to overcome the problems which are encountered. Historically, device interconnections have been made with aluminum or aluminum alloy metallization. Aluminum, however, presents problems with junction spiking which result from dissolution of silicon in the aluminum metallization and aluminum in the silicon. This problem is exacerbated with the small device sizes because the shallow junction is easily shorted and because the amount of silicon available to satisfy the solubility requirements of the aluminum metallization is only accessed through the small contact area, increasing the resultant depth of the spike. Adding silicon to the aluminum metallization has helped to solve this problem, but has, in turn, resulted in silicon precipitation and other problems.
A variety of solutions have been attempted to overcome the problems inherent with aluminum metallization. For example, the device region exposed through a contact opening has been metallized with chemical vapor deposited (cvd) tungsten. The tungsten is deposited by the reduction of WF.sub.6 with hydrogen. The WF.sub.6, however, initially reacts with the silicon surface, employing silicon from the contact area in the reaction. This results in what has become known in the literature as "worm holes" or tunnels in the silicon which can also cause shorting of the underlying junction. The use of sputtered instead of cvd tungsten would overcome the problem of reaction with the silicon, but sputtered tungsten is unable to reduce any native oxide film which may be present on the surface of the device region. The remaining oxide film results in high contact resistance. A further attempt at solving this problem has employed the use of a reactive silicide to make the initial contact to the device region. The silicide is then contacted with aluminum. The silicide makes a low resistance contact to the silicon, but the silicide-aluminum contact has the same problem as does the use of aluminum directly on the silicon. Silicon from the silicide and from the underlying junction dissolves in the aluminum, resulting in aluminum spikes through the underlying junction.
Another problem which is exacerbated by the shrinking device sizes is that of unreliable step coverage by the device metallization as it traverses steps in the device topography and especially as the metallization traverses into the contact openings. It is therefore especially beneficial if the contact metallization provides a relatively planar surface to which the interconnect metallization can be applied.
It is therefore an object of this invention to provide an improved device metallization.
It is a further object of this invention to provide an improved semiconductor device structure.
It is yet another object of this invention to provide an improved process for fabricating semiconductor devices.